Tekken Tag Tournament 2

Bob has a real abusive move, which is really simple and will get you through the whole game really easy. Pick him solo, so he has more health and you get this achievement. (Tag will get you the other achievement).

Hold Back and push both LP and RP (X+Y Default).

You'll do a combo starting with a headbutt and then two more hits, it's really powerful and will make short work of the Arcade mode.

Set your Options to Easy and 1 Round too. The final boss you'll have to do over 2 rounds but just use the combo.

Advanced:
To make the abuse even easier, you can use BACK then X then Y. This will punch them in the stomach twice and leave them keeled over ready for a Headbutt combo. Your all ready holding BACK, so just hit X+Y together.

As APassingShadow states, set the options to
Difficulty Easy
Rounds One

Offline
Arcade Battle

Those having problems defeating "unknown" in the last battle this is how I did it getting 2 perfect victories.

Choose solo Heihachi or Jin.
Keep backing off during the round (just hold back on the controller) every time your opponent comes toward you just tap B to kick. If you time this right you will knock your opponent to the floor each time and she won't be able to touch you. When she gets up and approaches again, just repeat. Do this until you are victorious!
Hope this is of some help!

How to beat the final final boss!

**SPOILERS CONTAINED BELOW**

Easiest way I've found in beating Unknown is using Jun and assigning LK + RK to any button. Simply spam the hell out of that move, you'll see variant moves happening naturally. This is how I beat Jun/Unknown first time.